The present invention generally relates to radio frequency (RF) communications and more particularly relates to a system and a method for receiving and processing RF communications.
In conventional communications systems, data is transmitted by modulating a carrier signal at a frequency fc using any one of a number of well known modulation techniques. After receipt by the receiver antenna, the modulated signal is typically passed through a low noise amplifier for improved sensitivity. The signal next is either upconverted or downconverted using one or more mixing stages to a desired frequency which can be more easily processed.
The mixing taking place during up- or down-conversion mixes the received signal with a local oscillator (LO) signal at a desired local oscillator frequency flo. The result is multiple intermediate frequency (IF) signals, for example one at fc+flo and another at fc−flo. One of these signals is the desired signal and the other is its image. The image, along with flo and various subharmonics resulting from the mixing are filtered out, for example using preselection filtering. While this type of system if suitable for narrow band applications, it is not practical in wide band applications where the image could fall in the band of interest and appear as a legitimate signal.
In some wideband applications, image reject mixers are commonly used. In an image reject mixer, the incoming signal is mixed with the local oscillator signal and in parallel with the local oscillator signal shifted by ninety (90) degrees.
Conventionally the in-phase component (I) and the quadrature component (Q) are typically combined so as to produce constructive interference for the signal of interest and destructive interference for the unwanted image signal. Although quadrature down-conversion can improve the image rejection of a receiver, each transconductance stage and the baseband stages that drive the transconductance stages have DC offsets that add in root mean square fashion possibly resulting in a relatively large LO feedthrough.